BANGKOK: Unlawful actions by Thai police and protesters threaten further violence in Thailand's ongoing political conflict, Human Rights Watch said today.

Thai authorities and protest leaders should take all necessary steps to end politically motivated violence and lawbreakers on all sides should be prosecuted.

On February 18, police attempts to forcibly retake a site held by protesters supporting the opposition People's Democratic Reform Committee escalated into a street battle in Bangkok. Four protesters and one police officer were killed by gunfire, according to the Erawan Emergency Medical Service.

At least 70 protesters and police officers were injured from gunshot and shrapnel wounds, beatings, and teargas inhalation.

''The bloody clashes between Thai police and protesters on February 18 have made a bad situation worse,'' said Brad Adams, Asia director at Human Rights Watch. ''Excessive force by the police and violence by groups on both sides of the political divide needs to stop to prevent this situation from escalating out of control.''

Thai authorities should promptly and impartially investigate violent incidents that since November have resulted in 16 deaths and 668 injuries, and prosecute all those responsible for criminal acts, regardless of political affiliation or rank.

The clash erupted as police sought to retake Phan Fah Bridge on Ratchadamnoen Road from anti-government protesters of the so-called Dharma Army, who are part of the PDRC's network.

After negotiations between the two sides collapsed, police baton-charged, threw teargas canisters, and fired rubber bullets at protesters. Police moved through the protest site and at approximately 10am arrested PDRC leader Somkiat Pongpaibul.

Shortly afterwards, police came under heavy gunfire from gunmen who appeared to be operating in tandem with protesters. A grenade thrown at the police line wounded more than 10 officers.

Four gunmen freed Somkiat from the police truck in which he was being detained. Police responded by firing pistols, shotguns, and assault rifles at protesters, using live ammunition.

Witnesses told Human Rights Watch that some protesters opened fire at a police minivan, wounding one officer, and dragged the driver out and beat him.

The clash finally stopped at around 3pm when police returned to the Metropolitan Police Bureau compound and the protesters retired behind their barricades.

Claims by anti-government groups that they are peaceful and unarmed were dispelled by their use of firearms during the February 18 violence, Human Rights Watch said.

This was the second time that armed gunmen operating with a high degree of skill and coordination were seen using military weapons in support of PDRC protesters.

The PDRC's secretary-general, Suthep Thaugsuban, and other protest leaders should take immediate action to identify their supporters who are using firearms or taking part in violence and turn them over to the authorities.

The Thai government should conduct a transparent and impartial investigation into the February 18 incident, Human Rights Watch said. This should include an inquiry into the decision by the security forces to fire live ammunition and the relevant circumstances.

The Thai government should be clear that its rules on using force comply with international human rights standards, and are strictly followed by the police at all times.

Previous clashes by opposing sides in Bangkok have also resulted in serious injury and death. On February 1, more than 20 gunmen supporting the PDRC, some with M16 assault rifles, engaged in a daytime street battle against members of the pro-government United Front for Democracy against Dictatorship, known as the ''Red Shirts,'' on Chaengwatta Road near Bangkok's Lak Si district. Seven people, including Red Shirt supporters, bystanders, and a photographer, were wounded in the fighting.

Human Rights Watch urged the Thai authorities to take appropriate action to quickly bring an end to the violence by all sides. They should conduct thorough, impartial, and transparent investigations into the recent violence and prosecute those responsible.

Since November, there have been more than 30 attacks against anti-government groups, members of the opposition Democrat Party, protest sites, and protesters' motorcades. The police have not reported any progress in the investigations of any of these cases.

Government measures to protect public safety may be justified so long as they are provided by law and are proportionate to the level of threat or legitimate objective to be achieved. The United Nations Basic Principles on the Use of Force and Firearms by Law Enforcement Officials provide that authorities shall, as far as possible, apply nonviolent means before resorting to the use of force and firearms.

The Principles provide that if the lawful use of force and firearms is unavoidable, then the authorities must use restraint and act in proportion to the seriousness of the offense. Lethal force may only be used when strictly unavoidable in order to protect life.

The Basic Principles also call for an effective reporting and review process, especially in cases of death and serious injury.

''In Thailand's deepening rights crisis, the government should demonstrate that no one can escape accountability for serious rights violations, no matter what their political position, reason or rationale,'' Adams said.

''Impunity for violence and rights abuses is a recipe for lawlessness and extended tit-for-tat violence. Accountability is critical for both government and protest leaders alike.''